1. Field of the Invention
The present invention relates to circuits for modulating an electron beam or for extracting power from a modulated electron beam. More particularly, it describes a system and method for creating an overmoded distributed interaction network comprising parallel or concentric grids.
2. Description of Related Art
The RF circuit of a microwave vacuum tube amplifier is used to modulate an electron beam and for extracting power from the modulated electron beam. For example, a typical klystron circuit includes a series of re-entrant cavities interacting with a beam propagating through an on-axis beam tunnel, or drift tube. FIG. 1(a) depicts a cross section through a klystron output cavity 104. Electron bunches 108 propagate through the drift tube along centerline 116 in the direction indicated at 110 and 112 from an electron source to a collector. The electron beam energy couples to the output cavity 104 at the location indicated by field lines 114. Beam energy may be extracted through a waveguide 106 or other coupling circuit. Another implementation is shown in FIG. 1(b). Reference designators in FIG. 1(b) that are the same as those in FIG. 1(a) refer to corresponding structures. Namely, electron bunches 108 propagate through the drift tube along centerline 116 in the direction indicated at 110 and 112 from an electron source to a collector. The electron beam energy couples to the output cavity 104 at the location indicated by field lines 114. Beam energy may be extracted through a waveguide 106 or other coupling circuit. In FIG. 1(b), grids 122 and 124 can be positioned across the drift tube noses of the klystron cavity 104, confining the RF electric field 120 to the gap region and thereby enhancing interaction efficiency. However, the accompanying interception of current by the grids 122 and 124 restricts average power capability. A conventional, doubly re-entrant klystron cavity operating in the fundamental mode is typically about one free-space wavelength in diameter. The beam tunnel and electron beam passing through the center of the cavity along centerline 116, however, are considerably smaller: the former is typically 0.1 to 0.2 wavelengths in diameter. This places a practical limit on the amount of beam current that can be focused through the beam tunnel, which in turn restricts the peak power of the device. Additionally, beam intercept by the RF circuit and, at higher frequencies, ohmic losses limit the average power capability. If the output circuit is configured so that the beam interacts with a higher order mode, an over-sized cavity can be used. While this may allow higher peak and average power operation, the interaction efficiency is substantially reduced. Accordingly, it would be useful to provide a system for extracting electron beam energy that overcomes many of these drawbacks of the prior art.